Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug insulator becomes coated with a foreign substance, such as fuel, oil, or soot. Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion until the spark plug is sufficiently cleaned or replaced. For example, the spark plug may be cleaned by burning off the soot accumulated on the fouled spark plug by operating the engine in speed-load conditions that sufficiently raise the spark plug tip temperature.
One example approach for spark plug cleaning is shown by Glugla et al in U.S. Pat. No. 8,132,556. Therein, based on the severity of the spark plug fouling, progressively aggressive actions are taken to burn off the accumulated soot. In particular, spark plug tip temperature is raised using a combination of spark timing advance, increase in engine load, increase in engine speed, etc.
However, the inventors herein have identified potential issues with such an approach. As one example, the various actions may conflict with engine commands for fuel economy. In particular, to improve engine performance, a controller may operate an engine with exhaust gas recirculation to slow combustion, and cool combustion temperatures in an attempt to reduce engine knock and NOx emissions. However, the use of EGR can result in cooler spark plug tip temperatures which makes it difficult for spark plug tip temperatures to be raised, despite the use of spark advance, to an operating temperature required for soot to be burned off from the plug. As a result, even with the use of aggressive spark advance, the spark plug may remain fouled. In addition, spark plug fouling can occur more frequently at lower speeds and light loads where EGR tends to be scheduled. The same problem may occur while the engine is a green engine, that is, while the engine is coupled in a vehicle at an assembly plant, before delivery to a customer. At the assembly plant, the vehicle may be started multiple times due to the vehicle being moved around to multiple lots. In addition, the vehicle may be started to test out engine components. The frequent engine operation generates excess soot which can foul the spark plug. However, the engine may not be operated sufficiently in speed-load regions that allow the spark plug to be warmed and the accumulated soot to be burned off. The cooler spark plug tip temperatures can exacerbate spark plug fouling issue.
In one example, some of the above issues may be at least partly addressed by a method for an engine coupled in a vehicle, comprising: reducing EGR by a larger amount when operating an engine at a vehicle assembly plant; and reducing EGR by a smaller amount when operating the engine responsive to a spark plug cleaning condition after the vehicle has left the assembly plant. In this way, spark plug fouling can be addressed more effectively.
As an example, while a vehicle is being operated and tested at an assembly plant, the engine may be operated with an adjusted EGR schedule. Specifically, since EGR cools combustion, EGR may be reduced by a larger amount for the green engine to enable spark plug tip temperatures to be maintained sufficiently high so as to reduce spark plug fouling issues. Likewise, if spark plug fouling is determined in the green engine, EGR may be reduced albeit by a larger amount. In comparison, when the vehicle is operated after leaving the assembly plant, the engine may be operated with a different EGR schedule responsive to an indication of spark plug fouling. Specifically, EGR may be reduced by a smaller amount to expedite spark plug cleaning. Once it is determined that the spark plug is sufficiently clean, or sufficiently warm, the initial EGR schedule (or a nominal EGR schedule) may be resumed.
The technical effect of adjusting EGR responsive to an indication of spark plug fouling is that EGR interference with spark plug cleaning is reduced. In particular, spark plug tip temperatures may be raised to, and held at, higher temperatures for a longer duration in a green engine (where the vehicle is still at the assembly plant), or a non-green engine (where the vehicle has left the assembly plant), improving spark plug health. By reducing EGR, at least transiently, when operating an engine with spark advance or increased engine speed/load to clean a spark plug, spark plug cleaning can be expedited without compromising the fuel economy and engine performance benefits on EGR usage. By better addressing spark plug fouling, cylinder misfire events can be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.